Efficiency Expert

The concept behind GM's new displacement on demand technology is simple: more power, less gas!

In describing GM's new fourth-generation small-block, we run the risk of sounding like that popular light beer commercial, "tastes great, less filling!" As silly as it sounds, the technology packed into GM's new Gen IV powerplant does allow one to have his proverbial cake and eat it too. Fuel efficiency and power are two goals high in any engineer's criteria; they also happen to be important to GM's customers. The down side is that they are usually at cross-purposes where one goal (fuel efficiency for instance) must be sacrificed for the other (power). Unfortunately, it's a compromise we're all too painfully aware of in our quest for performance.

We recently got a chance to sit down with Chris Meagher, assistant chief engineer of small-block truck engines at GM. And while we were initially intent on milking every last bit of information concerning the Corvette's new 400-hp LS2, we quickly realized that the real technology party is happening in the more utilitarian 5.3L version know as the LH6. As it so happens, the LS2 is a subtle evolution of the LS1, so we'll let the other magazines focus on it and we'll concentrate on the much-anticipated displacement-on-demand technology, which has managed to pass the LS2 by.

In our brief one-hour interview, we learned a lot about what the future of hot rodding will look like. In some respects, it looks very similar to what we've seen with the Gen III LS1 and LS6 engines, but in other ways, it's an evolutionary leap forward. Chris Meagher explained the impetus behind the redesign: "The design goal is pretty simple, we wanted to take the 5.3L V8 and make it more efficient. We wanted the customer to have the same characteristics of throttle response, power, general performance and towing capability that we had with the original LM7--and provide the customer with a more fuel-efficient package."

What GM Powertrain came up with--the 5.3L LH6--is almost identical to the outgoing LM7 on the surface, but technologically advanced on the inside. That technology enables cylinder deactivation (called displacement-on-demand or "DOD" by GM insiders) during periods of low load demand and is the source of the increased efficiency mentioned earlier. "We know that there are a couple of areas that we can attack to make an internal combustion engine more efficient," says Meagher. "These areas are mechanical efficiency, pumping losses, and unused heat energy. We knew that we could attack the area of pumping losses relatively easily with our displacement-on-demand system. That's the area this system addresses."

While typical hot rodding tricks revolve around making the engine bigger or getting more air and fuel inside an engine, the engineer working for the factory can't always afford the luxury of this approach. The working principle from the engineer's point of view is that the power available is what's left over after you take away everything else that robs power, such as friction and pumping losses. Meagher explains: "Pumping losses represent the work needed to bring a fresh charge into the combustion chamber and to expel the products of combustion. We have some tools that can simulate the vehicle performance characteristics relative to fuel economy. When we looked at mid-sized utility vehicles, we saw that we could achieve about an 8 percent increase in vehicle fuel economy."

To an engineer, eight percent is a huge improvement. At the gas pump or on the dyno, it's also a huge improvement. "Displacement-on-demand is a win-win for drivers who rely on the power of the small-block V-8 and are conscious about fuel economy. The implications of this technology are tremendous," says Meagher. For now at least, those implications will be exclusive to three vehicles, the 2005 Chevy TrailBlazer EXT, the GMC Envoy XL and Envoy XUV. But before moving on to the nuts and bolts of DOD, we'd like to point out that it has far-reaching possibilities for performance enthusiasts. Had GM elected to apply DOD technology to performance vehicles such as the Corvette, GTO or a future Camaro, there would be far less pressure from corporate average fuel economy (CAFE) to import sub-compacts from outside GM as is the case now.

The bottom line is, DOD on more cars could mean more--and better--choices for enthusiasts. As an example, an early prototype C6 Corvette with an LS2 running DOD provided equal power and acceleration to an LS2 without DOD, but produced 35 mpg instead of 30 mpg. When applied over a large volume of vehicles, GM could have the choice to pocket the improvement in economy and reduce the need to import small outside-sourced cars, or it could build a larger V-8 with more power (say a 6.5L V-8 with 430hp) and keep the same 30 mpg. In the end, for reasons not entirely understood by us but conceivably related to exhaust packaging, DOD technology did not make it into the C6 Corvette, GTO, CTSv or any other performance application as we had hoped. One thing Meagher did share with PHR is that the goal of DOD (on the limited range of SUVs currently planned) is to allow additional vehicle mass (in the form of increased content) without a commensurate decrease in fuel economy or performance.

All corporate politics aside, the engineers at GM Powertrain have designed yet another mechanical marvel, and it's all due to some remarkably modest changes to the very robust Gen III architecture on which the Gen IV is based. The new DOD-specific hardware includes two-stage switching lifters, a lifter oil manifold assembly (located in the valley of the engine), a redesigned lube circuit and oil pump, electronic throttle-by-wire operation, a pressure-activated muffler valve, and an improved E40 engine controller running DOD-specific software.

"In order to eliminate the pumping losses," says Meagher, "you need to disable both the intake and exhaust valve." This results in a completely sealed, deactivated cylinder, which is essentially an air spring being acted upon by a piston. Virtually all the work put into it during compression is returned to the crank during decompression, finally giving credence to the old joke about piston-return springs. (That's nothing. Wait 'til you hear about the muffler valve...)

"Currently, we could disable just the fuel delivery," says Meagher, "but the valves would still be opening and closing and each cylinder would still be doing work pumping air in and out. So there would be no net gain in efficiency--you wouldn't have eliminated the pumping losses at all."

In support of cylinder deactivation is some very interesting choreography from things ranging from throttle valve modulation to active exhaust tuning, but it all starts with the additional job tasked to the lifters. "We disable the valves through a device called a switching lifter," explains Meagher. "This differs from a normal lifter in that there is an inner body and an outer body connected by a spring-loaded pin. For V-8 operation, the pin is fully expanded by the spring so the two pieces act as one and the lifter acts like a regular lifter. When we want to disable the valve operation, we deliver high-pressure oil to a groove in the lifter that leads to the outside end of the pin, forcing the pin to collapse the spring. Now the two parts of the lifter are free to move relative to one another and as the cam lobe pushes on the follower the inner portion of the lifter pushes against another spring at the top of the lifter and does not transfer force to the pushrod."

A look at the lifter cross-section reveals an elegant, yet simple design that has the potential to change the way we think about traditional pushrod engines. (Ironically, when DOD is working, it hinges on lifters that do not lift! Something we never thought we'd ever want.) In order for the switching lifter to work effectively, the engine needed a redesigned oiling system. Both iron and aluminum versions of the engine block have redesigned oil galleries to support DOD oiling requirements. Those oil galleries are supplied by a lifter oil manifold assembly (LOMA) located in the lifter valley of the engine. Under cylinder deactivation, the LOMA routes oil to the applicable lifters by means of four lifter oil solenoids, which are controlled by a new E40 engine management controller. To supply the additional needs of the cylinder deactivation circuit, a higher capacity oil pump is fitted to the LH6 engine.

For the most part, the Gen IV engine family is very similar to the Gen III (LS1, LS6, LM7, LQ4, LQ9, etc.). Although the two are externally similar, there are several significant differences, which impede the interchangeability of some parts between Gen III and Gen IV engines. For one thing, the real estate required by the LOMA and its attendant electronics forced the relocation of the knock sensors and the camshaft position sensor. Since DOD relies on the use of electronic throttle control, the throttle body is not interchangeable with earlier cable-actuated throttle bodies. In concert with these DOD-specific changes, an improved coil-on-plug ignition system (which requires less energy), a returnless fuel system, and uprated LS6 cylinder heads (minus the hollow sodium-filled valves) have been employed. Fortunately, the cylinder heads do retain interchangeability between Gen III and Gen IV, which could prove to be a boon to older LS1s.

Readers who remember the 1980s will recall this isn't GM's first rodeo with cylinder deactivation. That first happened at Cadillac with the 8-6-4 engine, which was roundly criticized for its service record and its poor vibration (NVH) characteristics. The old adage of once bitten, twice shy applies here not only to potential LH6 customers, but also to the folks at GM working to make Gen IV the best engine architecture yet. GM's Meagher quickly points out the lessons learned: "I worked on the V-8-6-4 earlier in my career and [the LH6 is] the same idea. The key difference is the control system configuration. The key part of it that makes the transition imperceptible is electronic throttle control. Once the computer determines operating conditions are met to enable DOD, it uses engine vacuum as an indicator of customer power demand. When the computer decides to disable four cylinders, it calculates where the throttle needs to go such that the torque will be equal when you end up with four cylinders."

With the different modes of cylinder deactivation in the Cadillac 8-6-4, there was a dramatic change in NVH, and a corresponding difference in throttle response and exhaust tone. All of these were deemed unacceptable in a luxury car, and at the end of the day, the improved vehicle economy wasn't capable of offsetting the loss of comfort and power. In one sense, the failure of the Cadillac was a windfall to DOD engineers because the design obstacles had been clearly defined years ago.

In the LH6, the transition from eight-cylinder operation to four-cylinder operation is aided by electronic throttle control (ETC). At no time does the driver perceive a decrease in engine power when in V-4 mode because ETC applies a seamless increase in manifold pressure. (Translation: when the engine switches to four cylinders, your foot is still pressed the same amount on the gas pedal because the computer has opened the throttle more without you knowing it.) An increase in power demand is just as smooth; there is no dramatic surge in power during transition to V-8 operation beyond what is expected, that's because the ETC closes the throttle in conjunction with cylinder activation.

Meagher told PHR: "One area of dissatisfaction [with the Cadillac] was the transition feel when going from four to six to eight, or from eight to six to four cylinders. The reason that electronic throttle control helps that is that we are able to move the throttle with the computer, not the pedal. The computer moves the throttle blade without the customer knowing it. So the engine torque is the same on both sides of the transition event. You don't want the customer to know this transition has occurred."

As a side note, the LH6 does not employ a six-cylinder mode due to the unique vibration associated with it. One of the greatest complaints with the Cadillac was the excessive NVH in V-6 mode, a problem that has been completely avoided by transitioning directly between V-4 and V-8 modes. Nevertheless, GM Powertrain has designed tuned engine mounts for multiphase engine operation. The idea is that if a customer doesn't look at the window sticker, he will never know he has DOD in his vehicle.

In our interview with Chris, we remembered that there was one other mid-sized utility in the GM stable, which uses a 5.3L V-8, the Buick Ranier. That vehicle is not slated for DOD, although it will have the new LH6 engine (minus DOD). Chris's explanation for the exclusion of DOD in the Buick Ranier is too long to print here, but is worth a condensed look in light of the next point we need to make. GM deemed that in order for the customer to truly buy into the concept of DOD, it must be absolutely undetectable. Even with ETC and tuned engine mounts, there was a distinct difference in exhaust tone between V-4 and V-8 operation. To mitigate this difference, a pressure-activated valve in the muffler adjusts the exhaust path to deliver an appropriate amount of noise reduction. It was found that such an exhaust system had packaging limitations that precluded its use in the Buick Ranier, which has a shorter wheelbase than the vehicles currently slated to receive DOD.

Fortunately, The Ranier and any other non-DOD applications currently getting a 5.3L LM7 will still benefit from the non-DOD version of the LH6, which will replace the LM7. The LH6's freer-flowing LS6 cylinder heads, oiling system upgrades and ignition system upgrades beat the outgoing LM7 to deliver 290hp and 325 lb.-ft. of torque (in both DOD and non-DOD form). It's also worth noting that the upgrade to LS6 heads required a new piston to meet program requirements for compression and power. Strangely enough, that leaves only the GTO in the GM V-8 line-up as having the older, lesser-flowing heads of the LS1 (since LQ4, LQ9, LS6, LH6 and LS2 all have some variant of the improved LS6 head). GM, can we have the LS6 head for the GTO in the 2005 model?

Operationally, the LH6 always deactivates the same four cylinders in the firing order (1, 4, 6 and 7). According to Meaghan, lifter design and pushrod length are the same for all eight cylinders, but camshaft lobe profiles are different for the cylinders, which are deactivated. (This seems, in part, to contradict the GM media website, which states, "...in displacement-on-demand equipped engines, half of the cylinders have unique two-piece valve lifters..." -this being an important stipulation for those wanting to swap camshafts.)

For hot rodders wanting to modify their DOD-equipped LH6s, it's important to know that the switching lifter has a lift limitation of 15mm (at the valve). The factory cam uses 12.2mm of that (about .480 inch), giving the LH6 a theoretical valve lift limit of .590 inch. It's worth noting that this limit is for the lifter; a different valve spring would almost surely have to be used at this valve lift. Interestingly, it seems possible to grind a custom camshaft, which would only provide increased lift and duration to the non-DOD cylinders (2, 3, 5 and 8), thus allowing higher lift with standard non-switching lifters in those cylinders.

Before our interview ended, we asked one final question of Meagher: With four cylinders working the entire life of the engine and four cylinders working for approximately half that time, is there any extra maintenance or any deviation of maintenance from a normal V-8? To that Meagher says: "The service life of the engine will be the same as normal current engines. There are a couple of reasons for not making the service requirement any different for these four cylinders. One would be to avoid any confusion; the second, quite frankly, is that it's not necessary."

PHR would like to thank Tom Read of GM product communications for arranging the interview with Chris Meagher.

DOD JUNIOR

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The LH6 isn't the only engine that will receive the benefits of displacement-on-demand. We've already mentioned that Daimler-Chrysler's Hemi will be getting some form of DOD in the near future--but we don't have exact details of that yet. What we do know is that GM is coming out with a 3.9L V-6 (RPO code LZ8) which is scheduled to first appear in the 2005 Pontiac G6.

The "high value" 60-degree OHV V-6 will become the staple engine in vehicles which typically had the old 3100-, 3400- and more recent 3500-series engines (on which the LZ8 is based). Rated at 240hp at 5900 rpm and 245 lb.-ft. of torque at 2800 rpm, the 9.8:1 compression LZ8 will make 90 percent of its peak torque between 1800 and 5800 rpm. That compares very favorably with the venerable supercharged 3800 (RPO L67) which, in most iterations, makes 240hp at 5,200 rpm. What's more, the LZ8 does it with only 100cc more displacement, two valves per cylinder, pushrods and no supercharger.

Like the LH6, the LZ8 will have electronic throttle control and cylinder deactivation (running on three cylinders to the LH6's four), but will also throw into the mix variable valve timing and a variable intake manifold for dynamic runner tuning. Variable valve timing will be accomplished electronically by a gear-driven camshaft phaser capable of altering timing by as much as 40 degrees. As a happy coincidence, this feature will also allow the elimination of EGR control.

For the time being, the LZ8 will only be available in non-DOD form, but we've been assured that it will eventually appear. What's more, the LZ8 is designed to work in a rear-wheel drive configuration, which would pave the way for its use in a future base-model Camaro. If that occurs, the LZ8 would trump the '05 Mustang's base V-6 by nearly 40 horsepower while returning the same fuel economy.

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This diagram shows the difference between cylinder activation and deactivation in GM's new displacement-on-demand V-8. In the deactivated position, oil acts on a centrally-located release pin which collapses a spring, allowing the outer and inner lifter body to move independently. Although the lifter still accurately follows the cam lobe, the valve never opens.

The lifter oil manifold assembly (LOMA) provides high-pressure oil to the cylinder deactivation circuit, which runs through the cylinder case and to a groove on the lifter body. The LOMA sits in the lifter valley under the intake manifold.

To accommodate the additional needs of the LH6 oiling system, it is equipped with this higher-capacity oil pump. We can see lots of LS1/LS6 owners ordering these from dealerships for their next build-up!

The lube circuit for cylinder deactivation is shown in brown. When activated, oil flows from the LOMA into the block galleries and to the lifter body. The resulting cylinder deactivation provides an eight percent improvement in fuel efficiency.

In order to meet the compression target with the LS6 cylinder head, a new piston was required compared to the outgoing LM7. Note the oil-shedding anti-scuff coating on the skirt. This was an improvement made late in the Gen III's development.

The brains of the operation is the LH6's new E40 controller. Unlike previous PCMs which controlled both engine and trans, the E40 is dedicated to the LH6 and its hincreased appetite for computational power. In addition to standard engine housekeeping, the E40 must also control cylinder deactivation/activation, electronic throttle control and noise abatement circuitry.

This cut-away diagram shows many of the LH6's attributes, such as the position of the lifter oil manifold assembly (LOMA), a redesigned water pump, a stronger long-life timing chain, higher-capacity oil pump, electronic throttle control and returnless fuel system.

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